1. Field of the Invention
The present invention relates to an optical information recording method for recording information by holography, an optical information recording apparatus and an optical information recording medium which use the recording method. More specifically, the present invention relates to an optical information recording method which can efficiently record and fix information on recording material at a minimum exposure level over a short amount of time and which can impart excellent life to the recorded information, and to an optical information recording apparatus and optical information recording media which use the optical information recording method.
2. Description of the Related Art
An optical information recording method for recording information on an optical recording medium by holography generally utilizes optical interference inside the optical information recording medium between a reference beam and an information beam (object beam) with image information to thereby produce an interference image (interference fringes) that can be recorded on the optical information recording medium. An example of this kind of optical information recording method is a reflective optical information recording method shown in FIG. 7. With this method, information is recorded in the following procedure: The light from a first light source 61 is split into two beams, one forming an information beam 51 that passed through a half mirror 64, and one forming a reference beam 52 reflected by the half mirror 64. The information beam 51 is expanded through a mirror 66 and a beam expander 68, and is shined on the recording layer of the optical information recording medium 50. The reference beam 52 is expanded through a mirror 65 and a beam expander 67, and is shined on the opposite side of the recording layer. In this way the information beam 51 and reference beam 52 create an interference image, which is recorded on the recording layer as optical information. Even when the reference beam 52 is applied in the same direction as the information beam 51, an interference image is also created for recording, as in a case where the reference beam 52 is applied in a different direction. A technology in which the information beam 51 and reference beam 52 are applied in such a way that the optical axis of the information beam 51 is collinear with that of the reference beam 52 is called the “collinear technology.” An interference image is also created in this scheme, too. Reproduction of the recorded optical information is performed by applying another reference beam to the optical information recording medium from the direction for recording. In this way diffracted rays are created from the interference image and the optical information is reproduced by collecting them.
As a method for increasing the storage capacity of optical information recording media, there is proposed a multiplexing recording method for increasing the density of the interference images recorded. More specifically, the shift-multiplexing recording, angle-multiplexing recording, wavelength-multiplexing recording, and phase-multiplexing recording are utilized.
Among these recording methods, the shift-multiplexing recording has high compatibility with conventional CD and DVD recording methods where information is recorded on a spinning disc, and offers high random access performance. This is because in this recording method additional information, or interference image, is placed one after another on the initial recorded information (the initial interference image) while slightly moving either a beam spot or an optical information recording medium along the surface of the recording layer in a relative manner. For this reason, the shift-multiplexing recording is adopted for the collinear technology that uses a single lens for recording (see Nikkei Electronics (p 105-114) issued Jan. 17, 2005).
The shift-multiplexing recording method records new information on the previously recorded information; thus the greater the number of multiplexed interference images, so too does the exposure level for recording of another new interference image (information). For example, when a photopolymer recording material is used, the first interference image can be recorded at low energy. However, subsequent continuous exposure operations leads to a larger reacted (exposed) region to reduce the sensitivity of the recording material, resulting in high-irradiation energy to achieve as excellent recording quality as the first one.
This is because the reacted region inhibits the reaction of non-reacted (non-exposed) with beams, and/or because the amounts of photosensitive compositions (e.g., photopolymers) in the non-reacted region are reduced.
Since the exposed region with less number of recorded interference images requires less exposure level for recording of another new interference image in such a multiplexing recording method, reactions in the photosensitive compositions of such an exposed region have not proceeded fully and thus they need a fixing operation to complete such reactions. For this, for example, the following fixing method is proposed: After all recording operations are completed, the entire surface of an optical information recording medium is irradiated with a fixing beam to fix the recorded information (see Japanese Patent Application Laid-Open JP-A) No. 05-234855).
With this fixing method, however, the entire surface of an optical information recording medium is irradiated with a beam to cause reactions to take place even in a non-exposed region, making it impossible to record additional information on the non-exposed region. In addition, although fully-exposed recording region requires no fixing operations, they may be undesirably irradiated with the beam, leading to inefficient recording due to this excess irradiation.
Accordingly, so far there has not been provided any optical information recording method, optical information recording apparatus and optical information recording medium, which, upon exposure and fixing operations with the shift-multiplexing recording method, generate no excessively-exposed region, can efficiently fix the recording regions, and never adversely affect non-recorded regions.